This invention relates to an engine system including a multi-cylinder internal combustion engine which has a scavenging phase and more particularly to the construction of a multi-cylinder internal combustion engine using one cylinder as an air pump for the admission of scavenging air.
It is recognized that, in the conventional engine, a portion of exhaust gas tends to remain in each cylinder when the exhaust stroke has terminated and the amount of such residual gas will increase under partial load conditions, causing unstable engine operation under these conditions. Thus, if the residual gas is expelled from the cylinder with scavenging air and replaced with the air, the admission of more fuel could be effected and the probability of misfiring due to the presence the residual gas lowers. This makes it possible to improve engine power output and fuel economy.
To this end, it is known to admit scavenging air under pressure into the engine cylinders to forcibly expel the residual gas from the cylinder. An engine system embodying this known idea comprises an air pump which is driven in timed relationship with the engine r.p.m. to increase the amount of scavenging air in response to the engine speed because there is the tendency that the residual gas increases as the engine speed increases. The problem in this system, however, is that the air pump is drivably connected to the engine crankshaft through a complicated linkage to synchronize the air pump with the engine speed. Another problem is that there can not be found enough room in the engine compartment for accommodating the air pump and complicated linkage. Since the air pump which is capable of effecting the admission of air under sufficiently high pressure is expensive, this is also a problem.